The Cambridge OH Suppression Instrument
COHSI

COHSI stands for the Cambridge OHSuppression
Instrument.
COHSI was a special purpose instrument, designed for NIR low resolution
(R~500) faint object spectroscopy. Two very important technical successes
came out of this project:

1) OH masking technology - The optics required to deliver a high
resolution spectrum from 1.0-1.8 microns on to a mask surface worked extremely
well. We also developed in-house techniques for making a curved mask surface
with very low reflectivity (<0.5%) for the OH lines and very high reflectivity
(>98%) for the rest of the spectrum.

2) NIR lens-array, fibre coupled Integral Field Unit (IFU) -
We made a 100-element IFU using a macro-lens array. Each fibre was individually
positioned with respect to a lens in the array to a precision of ~2 microns
giving excellent IFU uniformity and throughput.

COHSI is now being used to construct the next generation spectrograph
CIRPASS which
exploits both of these technologies to the full.

COHSI was designed when the biggest detectors available were 256x256
arrays. The requirement was for a spectrograph that was not limited by
the OH emission but by the much fainter sky emission in between the OH
lines. To achieve OH suppression at a suppression mask (at R~3500) and
record the whole of the J or H band (at R~500) on the small detector
the light had to be dispersed, masked, fully undispersed and then finally
fed into a cryogenic spectrograph where the light was redispersed again.
Once larger 1024x1024 and 2048x2048 arrays became available it became possible
to capture the spectrum at high resolution without losing wavelength coverage
and so the COHSI approach lost its appeal because of its low throughput.
We are now therefore using the front end of COHSI (the IFU and the suppression
optics) and replacing the backend (the undispersing optics, the second
fibre bundle and the cryogenic spectrograph) with a simple camera which
uses a large array to image the mask surface. This new instrument configuration
is called CIRPASS.

COHSI was designed to suppress the nightsky airglow due to OH
emission in the 1.0-1.8 micron region by filtering out the OH using
masks.
COHSI was a fiber
fed instrument, which gave COHSI stability (the bulk of the instrument
sat on the observing floor) and telescope independence (a small change
of fore-optics in front of the fibers allowed COHSI to move from one telescope
to another).